CARNATION WILT DISEASES 11 



The incubation period is ordinarily long but under warm atmospheric condi- 

 tions the disease progresses rapidly. Bickerton (5) reported 92 percent infected 

 plants at a soil temperature range of 70°-80° F. in 23 to 45 days, and 39 percent 

 at 50°-65° in 56 to 75 days, following inoculation of the roots. Under the cool 

 conditions of the winter months, infection remains latent for some time. As 

 the disease appears among scattered plants abundant inoculum is provided for 

 secondary infections in subsequent stages of culture. Thus under favorable con- 

 ditions the amount of disease may run high among certain classes of stock. 



Rhizoctonia Stem Rot 



The Rhizoctonia stem rot fungus grows in the soil and attacks one plant after 

 another. Infection is readily induced by placing a culture of the fungus in contact 

 with the stem at the soil line and maintaining moist conditions. Infection is 

 more generally successful by direct insertion of bits of the fungus into incisions 

 in the plant (25). Pieces of Rhizoctonia from culture were inserted in scalpel 

 slits in the stems of Nina Brener carnations in June, 9 months after benching, 

 and in other instances the fungus was placed on the stem at the level of the soil 

 and swabbed with moist cotton. One of 9 inoculations on uninjured stems was 

 positive; 5 of 6 inoculations in scalpel slits in the stem were positive. 



Conditions which both favor the fungus and place the plant at a disadvantage 

 contribute to occurrences of the disease. Infection is readily induced by mixing 

 the fungus with the soil in which the plants are growing. Peltier (52, 53) re- 

 ported that the disease is most progressive at temperatures of 86°-88° F. and 

 soil moisture either too high or too low for the best growth of the plants, and he 

 considered wounding necessary for infection. The fungus enters through cracks 

 in the corky layer of the bark near the surface of the soil. More stem rot is ob- 

 served among deeply set plants branching below the soil surface than among those 

 with a single stem in the soil. Infection among old plants was high following 

 inoculation of the stem at wounds below the natural soil level (52). Destruction 

 of cuttings and young plants readily follows when the sand or soil is inoculated 

 with the fungus and when favorable temperature and moisture conditions are 

 provided. The fungus is widespread in all types of soil and more virulent in 

 some than in others. The occurrence of carnation stem rot is largely influenced 

 by conditions created by the grower, which are favorable to the fungus and 

 unnecessary for the best growth of the plants; and this was recognized early in 

 the history of American carnation culture. Serious cccurrences of the disease 

 even then were considered the result of deep planting, excessive soil moisture, 

 and imperfect drainage (40, 68). 



STOCK PLANTS AND THE SELECTION OF CUTTINGS 



The importance of selecting cuttings from healthy stock plants is generally 

 stressed in the literature. The risk of serious losses from ignoring this rule is 

 usually very great, particularly in relation to Alternaria blight, Fusarium root 

 rot, Fusarium branch rot, and carnation rust caused by the fungus Uromyces 

 caryophyllinus (Schr.) Wint. These pathogenic organisms exist in the foliage 

 and branches. Starkey (58, 59, 60) reported that at the end of 63 days 83.3 

 percent of the plants had died from Fusarium root rot as the result of propagating 

 cuttings gathered from diseased branches, and that only 38.1 percent of the 

 cuttings rooted. After 160 days only 7 plants survived. In contrast, 94 to 98 

 percent of the cuttings from healthy stock plants remained healthy. Wickens 

 (78) noted that healthy cuttings contaminated with the Fusarium root rot organ- 

 ism rapidly succumbed to basal rot. 



